The field of the present invention relates to semiconductor lasers. In particular, apparatus and methods are described herein for frequency stabilization of a semiconductor laser.
Semiconductor laser sources are prevalent light sources employed for fiber-optic telecommunications. Data is typically encoded on an optical carrier by intensity modulation of the laser output. Multiple data channels may be carried on a single optical fiber using wavelength division multiplexing (WDM) techniques, wherein multiple optical carrier wavelengths propagate simultaneously through the fiber, each independently modulated for encoding an independent data stream. Laser sources for such a system must therefore enable high-speed intensity modulation while maintaining a stable emission wavelength. Simple Fabry-Perot semiconductor lasers may be readily and inexpensively manufactured, but typically have relatively large spectral widths and/or relatively large wavelength/temperature coefficients. Distributed feedback semiconductor lasers (DFB lasers) and other single-longitudinal-mode lasers may be fabricated and operated with less variation in output wavelength, but are susceptible to feedback and/or interference from back-reflections from the optical transmission system and are difficult and expensive to manufacture. Precise matching of a longitudinal mode frequency to the reflectivity spectral profile of the DFB waveguide grating is also problematic, and can lead to undesirable mode hops. It should be pointed out that design requirements and/or constraints for semiconductor laser sources may be dictated in part by properties of previously-deployed optical fiber that will carry the laser output. Set forth hereinbelow are a variety of exemplary embodiments, and methods of fabrication, of semiconductor lasers including waveguide gratings adapted for wavelength stabilization of the laser output.